This invention relates to optical metrology of materials. More specifically, it provides a novel method and metrology tool for monitoring and characterizing thin films, such as low-k dielectric films.
As ultra-large-scale-integrated (ULSI) circuits continue shrinking in size to meet ever growing demand for higher performance, the industry is moving toward interconnect technology based on using Cu for metallization and low-xcexa materials for inter-layer dielectrics. The need for the latter stems from the fact that the capacitance of the interlayer dielectric has become an increasingly limiting factor on the overall performance of ULSI chips. Successful adoption of these newer materials and process technologies requires effective monitoring methods for various film structures and processes. The introduction of new interconnect technology also imposes new challenges and unique requirements. Accordingly, effects have been undertaken in the industry to develop metrology tools for these applications.
Take SiLK dielectric for example. This is a dielectric material under the trade-name by Dow Chemicals. Amongst various candidate materials for low-xcexa inter-layer dielectric (ILD) applications, SiLK appears to be one of the most promising. SiLK dielectric thin films can be readily produced by way of deposition using conventional spin-coaters. Like other spin-on candidate materials, mechanical, thermal, and electrical properties of SiLK thin films depend upon the degree of cure, during which a polymerization process is thermally activated. Hence, a precise control of the curing temperature and the curing time in real time is critical to the quality of the SiLK dielectric films thus produced.
Yang et al. in xe2x80x9cOptical Metrology for Monitoring the cure of SiLK Low-xcexa Dielectric Thin Filmsxe2x80x9d, Advanced Metalization Conference (AMC), Orlando, Fla. 1999, described a method for characterizing SiLK thin films by a combination of spectroscopic reflectometry and ellipsometry techniques. The authors found that at the wavelength of 314 nm, indices of refraction, n(xcex), of the SiLK thin films change systematically with the curing parameters such as the curing time and curing temperature.
A method that has proved to be particularly effective in characterizing the optical properties of thin films is the so-called xe2x80x9cnandkxe2x80x9d method. This is a measurement technique that simultaneously determines the thickness d, spectra of physical constantsxe2x80x94namely, index of refraction n and extinction coefficient k, energy band-gap Eg, and interface roughness "sgr" of thin films deposited on opague or transparent substrates. At the core of the xe2x80x9cnandkxe2x80x9d method are the original Forouhi-Bloomer dispersion equations for n and k, as described in U.S. Pat. 4,905,170. It is valid from the vacuum-ultra-violet (VUV) to the near-infra-red (NIR) range in spectrum. The high versatility and non-intrusive nature of the xe2x80x9cnandkxe2x80x9d method makes it applicable to a broad range of semiconductor, dielectric and thin metal films. An important advantage of the xe2x80x9cnandkxe2x80x9d method from a practical standpoint is that it can provide a variety of physical constants of a thin film by requiring only one measurement technique, such as reflectometry or transmittance, as opposed to two simultaneous techniques in the case of Yang et al. described above.
In the interconnect technology applications involving the use of Cu, the presence of copper oxides can degrade adhesion between copper and silicon nitride. Thus, the oxidation state of copper needs to be monitored. Given that there is a substantial difference between the optical properties of copper, cuprous oxide, and cupric oxide, the xe2x80x9cnandkxe2x80x9d method can be used to monitor the presence of either a single phase or mixture phases in copper.
As thin-film fabrication and characterization become an increasingly indispensable part of modern technology, there is a need in the art for a non-intrusive, sensitive and robust monitoring tool that can characterize a variety of thin-film processes in an accurate and real-time fashion.
The present invention provides a method for monitoring a modifying-process taking place in a thin-film sample and thereby characterizing the sample thus modified, wherein the modifying-process is introduced for purpose of improving physical properties of the sample. The present invention further provides a monitoring tool for characterizing various thin-film processes.
In the method of the present invention, a modifying-process is introduced to a thin-film sample; an observing spectral range is identified, in which one or more sensitive xe2x80x9cnandkxe2x80x9d parameters display pronounced changes in response to the modifying-process; and these sensitive xe2x80x9cnandkxe2x80x9d parameters are then used to monitor the modifying-process and thereby characterize the effects of the modifying-process on the physical properties of the sample.
In this specification and appending claims, a modifying-process should be construed as a process carried out in a thin-film sample for purpose of modifying and improving one or more physical properties of the sample. The xe2x80x9cnandkxe2x80x9d parameters include index of refraction n, extinction coefficient k, thickness d, energy band-gap Eg, and interface roughness "sgr" of a thin-film sample, as determined by the xe2x80x9cnandkxe2x80x9d method.
As a way of example, the modifying-process can be a curing process in a thermal-activated polymer film such as a SiLK dielectric film, controlled by the curing temperature and the curing time. The energy band-gap Eg and the spectra of extinction coefficient k=k(xcex) of a SiLK dielectric film exhibit pronounced changes with the curing temperature and curing time. The spectra of extinction coefficient K appears to be particularly sensitive to the curing temperature and curing time in the wavelength range of 320-370 nm. As such, the energy band-gap Eg and extinction coefficient K provide sensitive xe2x80x9cindicatorsxe2x80x9d of the underlying curing process taking place in a SiLK dielectric film.
The method of the present invention has advantages of being non-intrusive, real-time, sensitive, and versatile. Another inherent advantage of the present invention is that an assortment of the xe2x80x9cnandkxe2x80x9d parameters can be obtained by using only a single measurement tool, in contrast to two (or more) simultaneous measurement tools used in the prior art.
The novel features of this invention, its objects and advantages will become apparent from the following description and accompanying drawings.